The fastb file format is a compressed binary form of the fasta format for reads and contigs. (The 'b' in fastb stands for 'binary'.) In fastb, each of the four bases is denoted by two bits --
11 -- so that every byte can contain four bases, as in a basevector. Files in this format include reads.fastb and mergedcontigs.fastb.
Fastb is also the name of an executable module. It converts a file from fasta into fastb format. Note that the fastb format cannot store as much information as fasta; in particular, the letter N, representing gaps, has no parallel. When Fastb is run on a fasta file, Ns are converted to random bases, and (if FASTAMB=True) a fastamb file is created that contains the information that was lost.